The present invention relates to a method for producing a ceramic diaphragm structure which is used as a constitutional member in various kinds of sensors, piezoelectric/electrostrictive actuators, or the like.
A ceramic diaphragm structure has a structure in which a thin and flexible diaphragm plate is superposed on a substrate having at least one window portion so as to cover the window portion and to work as a diaphragm. Such a ceramic diaphragm structure is used for various kinds of sensors by constituting so that a diaphragm portion detects a bending displacement originated from a subject to be measured by an adequate means, or used as a constituting member of a piezoelectric/electrostrictive actuator by giving a pressure to the pressure room formed inside the actuator by deformation of the diaphragm portion.
A ceramic diaphragm structure is produced by unitarily superposing a thin ceramic green sheet (hereinafter referred to as a green sheet) on a ceramic green substrate (hereinafter referred to as a green substrate) to obtain a laminate, and subsequently firing the laminate. After firing, the green substrate becomes a substrate, and the green sheet becomes a diaphragm plate. In recent years, there has been used a ceramic diaphragm structure 3 which has a diaphragm portion 1 having a protrudent shape toward the side opposite to a window portion 8 of a substrate 2 as shown in FIG. 1, so as to prevent a crack or a depression in a firing step. Such a diaphragm portion 1 having a protrudent shape as described above can have a higher inherent resonance frequency in comparison to a diaphragm portion having a flat shape. Further, it is recognized that a diaphragm portion having a protrudent shape is excellent in mechanical strength, and mechanical strength is not hindered upon sintering a film formed on the surface of the diaphragm portion 1.
When a ceramic diaphragm structure having a protrudent shape is produced, there are used materials for a green substrate and a green sheet, i.e., materials within a range shown by a slanting line in FIG. 2, which satisfies the following formulae 1), 2), and 3). EQU S(substrate)-S(sheet).gtoreq.-0.08(T.sub.70 (substrate)-T.sub.70 (sheet)}-11) EQU 0.ltoreq.T.sub.70 (substrate)-T.sub.70 (sheet).ltoreq.300 2) EQU S(substrate)-S(sheet).ltoreq.20 3)
S(substrate) and S(sheet) denote shrinkage rates (%) of the green substrate and the green sheet, respectively. T.sub.70 (substrate) and T.sub.70 (sheet) denote mid-sintering temperatures of the green substrate and the green sheet.)
Japanese Patent Laid-Open 8-51238 discloses that by using such a material, a protrusion can be made in a green sheet toward the side opposite to a window portion in a substrate during firing without any crack or the like. That is, a thin ceramic portion can be formed by setting differences in shrinkage rate and mid-sintering temperature between a green substrate and a green sheet.
Incidentally, a shrinkage rate (%) means a shrinkage rate (%) of a green substrate and a green sheet independently fired at the same temperature as firing a laminate in a direction of a surface, and the shrinkage rate (%) is shown by {(length before firing-length after firing)/length before firing}.times.100(%). A direction of a surface is not the direction of thickness, and it means a predetermined direction on the surface where a green substrate or a green sheet is molded. A mid-sintering temperature means a firing temperature at which a shrinkage reaches 70% of the aforementioned shrinkage rate, S(substrate) and S(sheet) in a firing step, and a mid-sintering temperature is a barometer showing a sintering speed.
However, the method disclosed in Japanese Patent Laid-Open 8-51238 is on the supposition that a shrinkage rate and a mid-sintering temperature of a green substrate are even from a portion near a green sheet to a portion apart from the green sheet. In this method, a diaphragm structure has a large waviness, and a warpage is caused wholly in a ceramic plate including diaphragm structures. As shown in FIG. 5 and FIG. 7, a ceramic diaphragm structure 3 is constituted of a substrate 2 and a diaphragm plate 12. A plurality of the ceramic diaphragm structures 3 constitutes a ceramic plate 15. It is difficult to reform the aforementioned warpage and waviness even if the ceramic plate 15 is subjected to firing again with loading. When a load on the ceramic plate 15 is too large, a diaphragm portion and/or a substrate damage(s). When a warpage or a waviness is left as it is, a dimensional preciseness of a diaphragm structure deteriorates, and therefore, a preciseness of printing of a pattern on a diaphragm plate deteriorates, and/or a variance of a thickness of a film formed on the diaphragm plate is generated. Accordingly, when such a diaphragm structure is used for a sensor, the sensor has a variance of detection preciseness; and when it is used for a piezoelectric/electrostrictive actuator, the actuator has deterioration or variance of displacement.
Accordingly, the present invention aims to provide a method for producing a ceramic diaphragm structure, which can form a diaphragm portion having a protrusion toward the side opposite to a window portion of a substrate and which can advantageously minimize a waviness of a diaphragm structure and/or a warpage of a ceramic plate including diaphragm structures.